The present invention relates generally to apparatus and systems for condensing water and solvent from gas utilizing heat exchangers.
More particularly, the present invention relates to a gas condenser apparatus having a plurality of heat exchangers in which water and other contaminants such as organic solvents are condensed from exhaust gas. The condenser apparatus includes an automatic defrost cycle control for alternately routing the exhaust gases from an input heat exchanger through one of two output heat exchangers while the other output heat exchanger is being defrosted. In one preferred form of the invention, the heat exchangers are each of the counterflow type. In an alternate form of the invention, the output heat exchangers are refrigeration coils. Of course, other types of heat exchangers (i.e. cross flow heat exchangers, parallel flow heat exchangers, heat pipe exchangers, and heat wheels) are suitable as well. The invention further relates to method and system apparatus for reducing oxygen from system gas within an industrial dryer or process oven in which water and solvents are vaporized by recirculating a small portion of the system gas through a condenser apparatus.
Air pollution regulations promulgated by the Environmental Protection Agency of the United States Government require that the amounts of contaminants introduced into the atmosphere from certain manufacturing operations be below specified levels. For example, industrial dryers or process ovens utilized in connection with certain coating processes, e.g. assembly line automobile painting, magnetic tape manufacture, fabric coating, certain printing processes (flexographic and rotogravure) and drying of automobile vinyl, typically involve the vaporization of certain organic solvents such as kerosene, acetone, toluene, and alcohols. In many instances exhaust gas from such equipment must be processed to reduce the concentrations of such organic solvents before the gas can be vented into the atmosphere. In addition, the ever increasing cost of such chemical solvents has made it highly desirable to condense them from the exhaust gas for reusage.
Another problem is that vaporized organic solvents present a significant danger of explosion when present at certain concentration levels. For a given temperature, pressure, and oxygen concentration in an atmosphere, a concentration of vaporized organic solvents below a minimum level called the lower explosive limit (LEL) cannot be ignited. Above a certain maximum vaporized organic solvent concentration called the upper explosive limit (UEL) the atmosphere will also not ignite. The insurance industry often requires that such equipment be operated below a certain maximum percentage of the LEL, e.g. fifty percent, in order to safeguard against explosion. However, minimum capital expenditure as well as the most energy conserving recovery approach depends upon processing exhaust gas heavily laden or having a high concentration of vaporized solvents through a condensing apparatus or some other type of recovery system.
Presently three basic methods are commonly utilized to deal with vaporized organic solvents. A first method involves heating the solvent laden exhaust gas to approximately 1400.degree. F. in order to burn away the solvents. In addition to destroying the solvents, this approach consumes a large amount of fuel to heat the total body of air being treated to 1400.degree. F. Large amounts of carbon dioxide, nitrogen and an appreciable amount of water are liberated. A second method involves passing the exhaust gas across a series of refrigerant cooling coils to eventually condense the vapor. This method requires substantial amounts of energy to operate the cooling coils. A third method, and heretofore believed the most commonly used method, utilizes charcoal bed filtration. The exhaust gas is passed through large flat beds of activated charcoal. Dual beds are used with one being reactivated with live steam and drained while the other is collecting solvent and water. The relatively expensive activated charcoal must be periodically replaced.
Previously, it has been known to use counterflow type heat exchangers as condensers. However, the cooling fluid has been a gas or liquid separate from the gas being cooled, as shown in U.S. Pat. No. 3,827,343 and U.S. Pat. No. 2,169,054. U.S. Pat. No. 3,232,029 discloses a condenser apparatus having a pair of heat exchanger stages for condensing a vaporized organic solvent out of an incoming gas in the first stage, the remaining gas being heated in the second stage. Condensed solvent is collected and drained off from the first heat exchanger stage through an outlet. The gas flowing through the heat exchanger stages comes into contact with coils through which a refrigerated coolant flows countercurrent to the flow of the gas. Due to the heat transferred during the condensation of the vaporized solvent, the coolant entering the coils of the second heat exchanger stage is at a higher temperature than the gas passing through the second stage. Thus, the second heat exchanger stage heats rather than cools the gas passing therethrough. In none of these patents is the cooling fluid provided by feeding back the gas being treated after condensation of water vapor or solvent from such gas.
British patent specification No. 711,067 shows a condenser system employing two heat exchanger stages for cooling. However, the second stage is employed for cooling the cooling liquid used in the first stage. In addition, the cooling liquid is separate from the treated gas which is condensed in the first stage. In none of the above discussed patents is water condensed in one heat exchanger stage while vaporized solvents are condensed in a second heat exchanger stage to separate condensed water and solvent.
Condenser apparatus often collect frost from the water fraction normally found in ambient air in their heat exchangers. This frost buildup tends to obstruct the flow of gas through the heat exchangers and results in a lowering of heat exchange efficiency. This in turn reduces solvent recovery efficiency in those condenser apparatuses designed to handle solvent laden exhaust gas. None of the patents discussed above discloses or suggests the use of an automatic defrost cycle control system to selectively connect one of two heat exchangers into the gas flow path while the other is defrosted, for eliminating frost buildup from a heat exchanger to improve the air flow characteristics and heat exchange efficiency thereof.
In addition, U.S. Pat. No. 3,798,787 of Heen is understood to show a condenser system operating in a nitrogen charged paint dryer environment which employs, in succession, first cooling coils, a cross flow heat exchanger, second cooling coils and perforate plates surrounded by third cooling coils. The second and third coils are in a common chamber and supplied with refrigerant from a single compressor so that they apparently will be operating substantially at the same temperature. Also, Heen states that the second coils include alternately operating halves with one half being de-iced while the other is condensing and vice versa. Additionally, cooled gas from the chamber is fed back through the heat exchanger for cooling purposes. However, Heen differs from the present invention for any one of a number of reasons. For example, because water is not condensed in the upstream heat exchanger, heavy icing will apparently occur at the second set of coils making the system inefficient. Also, separation of water vapor and solvent condensed in the chamber of Heen will be difficult because the second coils apparently will condense both water and significant amounts of solvent together. Also, in Heen, efficient heat transfer within the heat exchanger is impaired because the first cooling coils pre-cool the gas prior to its entry into the heat exchanger and exposure therein in heat transfer relationship to cooled gas from the chamber.
Therefore, a need exists for a more efficient solvent condenser apparatus and solvent drying oven.